What Does Failover Mean?
Failover is the constant capability to automatically and seamlessly switch to a highly reliable backup. This can be operated in a redundant manner or in a standby operational mode upon the failure of a primary server, application, system or other primary system component.
Techopedia Explains Failover
Failover for a server uses a heartbeat cable connecting two servers. As long as the pulse or heartbeat continues between the two servers, the secondary server will not initiate its instances. However, any change in the pulse of the primary server will trigger the secondary server to take over the work of the primary and send a message to the technician or data center, which will then be responsible for bringing the primary server back online. Alternately, some systems simply alert the technician or center personnel, who then manually initiate the change to the secondary server. This is called an automated with manual approval configuration.
